Improve documentation

[patacca]

Improve the documentation both visually and content-wise.
List of changes:

• Switch to the theme insipid (compatible with Sphinx >= 7.2)
• Impose requirements over sphinx: only versions >= 7.2.4 but not 7.2.5 because of Leverage importlib.reload for reloading modules sphinx-doc/sphinx#11679 (also look at Sphinx 7.2.5 breaks with PyO3: "modules may only be initialized once per interpreter process" sphinx-doc/sphinx#11662).
  Hopefully we will be able to switch to sphinx 7.2.6 soon

There is still one major compatibility problem with sphinx-tabs (required by enum-tools[sphinx]). For some reason sphinx-tabs requires docutils 0.18.* but most of the other packages specifically avoid 0.18.* and 0.19.* as they are known to be buggy. Right now everything we are using is compatible with docutils 0.20.* except from sphinx-tabs.
I've checked and sphinx-tabs doesn't seem to complain if we force the installation of docutils 0.20.*
There is an open request to remove the strict (useless?) requirement of docutils from sphinx-tabs executablebooks/sphinx-tabs#171 but since they will address the issue we will still have to rely on manually forcing the installation of docutils 0.20.*

Example of a standard installation:

$ pip install -r requirements.txt
....
$ pip install enum-tools[sphinx]
... Some errors will occur ...
$ pip install docutils==0.20.1
... Warning for the incompatibility with sphinx-tabs ...
$ # Safe to use now